Conventionally, a variety of types of solid-state imaging devices comprises an MIS light-receiving device which has a photoelectrical conversion portion and a signal storage portion. One solid-state imaging device, using a MIS light-receiving device, has the MIS light-receiving device comprising a photoelectrical conversion portion and a signal storage portion and has an internal amplifying function. As an example of the type described above, a solid-state imaging device which uses a light-receiving device called a CMD (Charge Modulation Device) is disclosed by the inventor of the present invention in Japanese Patent Laid-Open No. 84059/1986 and is described in the thesis "A NEW MOS IMAGE SENSOR OPERATING IN A NON-DESTRUCTIVE READOUT MODE" on pages 353 to 356 of pre-published thesis for the International Electron Device Meeting (IEDM) held in 1986.
The cross-sectional structure of a CMD light-receiving device which forms a pixel of a solid-state imaging device of the conventional type is shown in FIG. 1. In FIG. 1, reference numeral 1 represents a p.sup.- substrate and reference numeral 2 represents an n.sup.- epitaxial layer which is formed on the p.sup.- substrate 1 and which forms a channel layer. Reference numeral 3 represents an n.sup.+ source layer or drain layer diffused in the epitaxial layer 2. Reference numeral 4 represents an n.sup.+ drain layer or a source layer which is also diffused in the epitaxial layer 2. Reference numeral 5 represents a gate insulating film disposed over the epitaxial layer 2, and reference numeral 6 represents a gate electrode which is disposed above the epitaxial layer 2 with the gate insulating film 5 therebetween. Reference numeral 7 represents a protection film (passivation film) of an insulating material formed over the gate electrode 6.
The operation of receiving light by means of a CMD light-receiving device having the structure of the type described above will now be described. When light 8 is incident from a position above the gate electrode 6, the incidental light 8 is introduced into the channel layer 2 through the protection film 7, gate electrode 6, and the gate insulating film 5. Hole-electron pairs are generated in the channel layer 2. The holes of the hole-electron pairs are stored in a boundary plane between the gate insulating film 5 and the n.sup.- channel layer 2. The boundary plane is positioned immediately beneath the gate electrode 6 to which a reverse bias is applied. As a result of this bias, the potential at the surface of the n.sup.- channel layer 2 rises, causing the potential barrier against the electrons present between the source layer 3 and the drain layer 4 to drop, and causing the electron current to pass through the n.sup.- channel layer 2. An optical amplified signal can be obtained by reading this current flow.
In the CMD light-receiving device having the structure described above, the incidental light 8 is introduced into the n.sup.- channel layer 2 through the protection film 7, gate electrode 6, and the gate insulating film 5. During this time, reflections and absorptions are generated by multi-interference effects in the multilayer film structure formed by air/protection film/gate electrode film/gate insulating film/n.sup.- channel layer. As a result, part of the incidental light vanishes. This phenomenon deteriorates the sensitivity of the light-receiving devices.
Therefore, in the conventional light-receiving devices, the thickness of each film forming the above-described multilayer film structure must be made the most suitable value for the purpose of improving sensitivity. For example, a case will now be described in which the gate electrode is formed by a polysilicon film with the use of an Si-substrate, and a thermally oxidized SiO.sub.2 is used as the gate insulating film. It is known that when the film thickness of the polysilicon of the gate electrode is set to 400 .ANG. to 800 .ANG., good transmissivity is shown when the film thickness of the SiO.sub.2 which forms the gate insulating film is 1000 .ANG. or less, and in the vicinity of 1500 .ANG., 3400 .ANG., and 5100 .ANG..
However, so long as the polysilicon is used for the gate electrode and the thermally-oxidized SiO.sub.2 film is used for the gate insulating film, the improvement in the transmissivity of light is limited. This fact will now be briefly described. That is, in general, reflectance R at the boundary plane between layers of different refractive indexes is expressed by the following equation, where that the refractive indexs are n.sub.1 and n.sub.2, and n.sub.1 &gt;n.sub.2 : ##EQU1## As can clearly be seen from this equation, as the ratio of n.sub.1 to n.sub.2 increases, the reflectance R increases.
In the light-receiving device whose structure is described above, a SiO.sub.2 film is often used for the protection film, while a thermally-oxidized SiO.sub.2 film is generally used for the gate insulating film. The gate electrode is formed by a thin polysilicon film. In this case, the refractive index of the polysilicon is up to 4, while the refractive index of the SiO.sub.2 is 1.45. Therefore, the ratio of the two refractive indexes is large, which increases reflection when multi-interference occurs, and reduces transmissivity.
An example of light transmissivity is shown in FIG. 2 in which the results of calculations of transmissivity of light which is in the visible radiation range (wavelength: 400 to 700 nm) is shown, assuming that the protection film is formed by SiO.sub.2, the thickness of the same is 24000 .ANG., the thickness of the polysilicon film of the gate electrode is 600 .ANG., and the thickness of the gate insulating film formed by SiO.sub.2 is 350 .ANG.. As can clearly be seen from this figure, the transmissivity of light in the vicinity of 600 nm is decreased by the reflection of light from the multilayer film.